Use a lightning rod and a heavy core to craft the copper mace.
Watch the video to see it in action:
Crafting
Craft the copper mace with a lightning rod and a heavy core.
Charging
When holding the copper mace during a thunderstorm, it has a chance to be charged.
The heavy core on the mace will light up to indicate it's charge state.
Each charge will give 4 lightning strike attacks.
Lightning Strike
Hit any mob with the copper mace when charged to deal a lightning strike attack.
All nearby mobs will be struck by lightning.